1. Technical Field
This invention relates generally to phase change memories.
2. Description of the Related Art
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory application. One type of memory element utilizes a phase change material that is electrically switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power.
FIG. 1 shows the scheme of a phase change memory 100. Memory 100 includes an array of n×n memory cells 111-119, each including a select device 120 and a memory element 130. Memory 100 may have any number of memory cells.
Memory elements 130 comprises a phase change material, thus memory 100 may be referred to as a phase change memory. A phase change material is a material having electrical properties (e.g. resistance, capacitance, etc.) that may be changed through the application of energy such as, for example, heat, light, voltage potential, or electrical current. Examples of a phase change material may include a chalcogenide material.
A chalcogenide alloy may be used in a memory element or in an electronic switch. A chalcogenide material is a material that includes at least one element from column VI of the periodic table or a material that includes one or more of the chalcogenic elements, e.g., any of the elements of tellurium, sulfur, or selenium.
Memory 100 includes column lines 141-143 and row lines 151-153 to select a particular memory cell of the array during a write or read operation. Column lines 141-143 and row lines 151-153 may also be referred to as address lines since these lines may be used to address memory cells 111-119 during programming or reading. Column lines 141-143 may also be referred to as bit lines and row lines 151-153 may also be referred to as word lines.
Memory elements 130 are connected to row lines 151-153 and are coupled to column lines 141-143 via select device 120. While one select device 120 is depicted, more select devices may also be used. Therefore, when a particular memory cell (e.g., memory cell 115) is selected, voltage potentials are applied to the column line (e.g., 142) and row line (e.g., 152) associated with the memory cell 115 to apply a voltage potential across it.
Series connected select device 120 is used to access memory element 130 during programming or reading of memory element 130. A select device is an ovonic threshold switch that is made of a chalcogenide alloy that does not exhibit an amorphous to crystalline phase change and which undergoes rapid, electric field initiated change in electrical conductivity that persists only so long as a holding voltage is present. Select device 120 operates as a switch that is either “off” or “on” depending on the amount of voltage potential applied across the memory cell, and more particularly whether the current through the select device exceeds its threshold current or voltage, which then triggers the device into the on state. The off state is a substantially electrically nonconductive state and the on state is a substantially conductive state, with less resistance than the off state. In the on state, the voltage across the select device is equal to its holding voltage VH plus I×Ron, where Ron is the dynamic resistance from VH.
For example, select devices 120 have threshold voltages and, if a voltage potential less than the threshold voltage of a select device 120 is applied across select device 120, then at least one select device 120 remains “off” or in a relatively high resistive state so that little or no electrical current passes through the memory cell and most of the voltage drop from selected row to selected column is across the select device. Alternatively, if a voltage potential greater than the threshold voltages of select device 120 is applied across select device 120, then the select device 120 “turns on,” i.e., operate in a relatively low resistive state so that electrical current passes through the memory cell. In other words, select device 120 are in a substantially electrically nonconductive state if less than a predetermined voltage potential, e.g., the threshold voltage, is applied across select device 120. Select device 120 is in a substantially conductive state if a potential greater than the predetermined voltage potential is applied across select device 120. Select device 120 may also be referred to as an access device, an isolation device, or a switch.
Each select device 120 comprises a switching material such as, for example, a chalcogenide alloy, and may be referred to as an ovonic threshold switch, or simply an ovonic switch. The switching material of select device 120 is a material in a substantially amorphous state positioned between two electrodes that may be repeatedly and reversibly switched between a higher resistance “off” state (e.g., greater than about ten MΩ) and a relatively lower resistance “on” state (e.g., about one thousand Ohms in series with VH) by application of a predetermined electrical current or voltage potential. Each select device 120 is a two terminal device that has a current-voltage (I-V) characteristic similar to a phase change memory element that is in the amorphous state. However, unlike a phase change memory element, the switching material of select device 120 does not change phase. That is, the switching material of select device 120 is not a programmable material, and, as a result, select device 120 is not a memory device capable of storing information. For example, the switching material of select device 120 may remain permanently amorphous and the I-V characteristic may remain the same throughout the operating life. A representative example of I-V characteristics of select device 120 is shown in FIG. 2.
Turning to FIG. 2, in the low voltage or low electric field mode, i.e., where the voltage applied across select device 120 is less than a threshold voltage (labeled VTH), select device 120 is “off” or non-conducting, and exhibits a relatively high resistance, e.g., greater than about 10 megaOhms. Select device 120 remains in the off state until a sufficient voltage, e.g., VTH, is applied, or a sufficient current is applied, e.g., ITH, that switches select device 120 to a conductive, relatively low resistance on state. After a voltage potential of greater than about VTH is applied across select device 120, the voltage potential across select device 120 drops (“snapback”) to a holding voltage potential, labeled VH. Snapback refers to the voltage difference between VTH and VH of a select device.
In the on state, the voltage potential across select device 120 remains close to the holding voltage of VH as current passing through select device 120 is increased. Select device 120 remains on until the current through select device 120 drops below a holding current, labeled IH. Below this value, select device 120 turns off and returns to a relatively high resistance, nonconductive off state until the VTH and ITH are exceeded again.
Processes for manufacturing memory cells of the above discussed type are known, but they are susceptible of improvement.